Peripheral blood neutrophils store large quantities of the serine proteinases elastase (HNE) and cathepsin G (catG) in catalytically active forms where they await mobilization in response to secretory and phagocytic stimuli. The proteinases are either released into phagocytic vesicles where they are thought to kill and digest microbes, or they are released into the extracellular environment where they are capable of degrading most connective tissue proteins. The proteinases are thought to be involved in the development of the inflammatory response by degrading pathogens and liquidizing tissues as a prelude wound healing. Frequently, though, neutrophil proteinases may escape normal control mechanisms and cause chronic, or sometimes acute, damage to connective tissues and humoral proteins. We have recently shown, by molecular cloning studies and biosynthetic radiosequencing techniques, that HNE and catG are synthesized as inactive zymogens in a neutrophil precursor-like cell line. Activation takes place about one hour after onset of synthesis and is caused by proteolytic removal of peptides at the N-terminus, or, less likely, at the C-terminus of the zymogens. The activation event represents a point at which the expression of proteolytic activity may be controlled, and we may be able to influence this to delineate the role of the proteinases in the biological activity of the neutrophil. The specificity of zymogen activation is unusual and does not correspond to other known serine protease zymogen activators, although the mechanism is probably the same. Our plan is to describe the specificity of zymogen activation by experiments on recombinant-derived HNE and catG mutants. We will then characterize the protease(s) responsible for zymogen activation, thereby generating tools to influence the process in neutrophil precursors. Knowledge of the mechanism of activation of HNE and catG will enable us to manipulate an important control point in the expression of the biological activity of the proteinases. We should thus be able to better understand the processes of neutrophil development, and gain insights into similar events that result in the inflammatory activity of mast cells and the tumoricidal activity of cytotoxic T-lymphocytes, both of which are hypothesized to contain protease zymogens with an activation specificity identical to catG.